There is much demand in industry for compact measurement systems and techniques for measuring the reflectance of devices, such as computer chips, flat panel displays, magnetic media used in data storage, and the like, during their manufacturing.
In many optical applications, there is often a need for designs having a high degree of compactness. For example, U.S. Pat. No. 5,024,493 to Fantuzzi et al. discloses a highly compact folded infra-red optical system having an effective focal length of about 240 mm and which employs three fold mirrors to fit the system into a package having a diameter of approximately 150 mm. Such compactness of design is technically challenging, but can result in significant advantages. In the present case, it enables accurate, spectroscopic, reflectance measurements of a sample where such sophisticated equipment could not previously be implemented due to space limitations. Such compactness can also result in cost-savings where space is at a premium.
Present day metrology instruments, including those using reflectometer systems, tend to be large and physically separated from the processing tools used to fabricate the devices. These processing tools are often in clean rooms, where air quality, temperature and humidity are tightly controlled. Conventional metrology instruments require 1.4-2.0 square meters of this costly clean room floor space. In addition, these metrology instruments are physically separated from the processing tools, so samples have to be removed from the processing tools and transferred to the metrology instruments. Often the environment inside the processing tool is designed to be even cleaner than the surrounding area. In such a case, the wafer must be placed in a sealed, airtight container while it is transferred. This takes time, causing costly delays in the production line.
Therefore, it is advantageous to integrate these metrology instruments into the processing tool. However, these processing tools are already designed to be as compact as possible because of the above mentioned cost of the clean room facilities in which they are housed. Thus, the size of an integrated metrology instrument is the determining factor in its ability to be integrated into a processing tool. At the same time, an integrated metrology instrument must provide highly accurate measurements of the samples in order to provide useful information on the performance of the processing tool. In addition, an integrated metrology instrument must provide these measurements quickly so as not to slow the production line.